A group of Iranian protesters have demanded Iran's Supreme Leader Ayatollah Ali Khamenei step down after Tehran said its military mistakenly shot down a Ukrainian plane, killing all 176 people on board.
Others took their frustrations to Twitter, asking why the plane was even allowed to take off when tensions in Iran were so high.
"Commander-in-chief [Khamenei] resign, resign," videos posted on Twitter showed hundreds of people chanting in front of Tehran's Amir Kabir University on Saturday.
Earlier on Saturday, Iran said its military had shot down the Ukrainian plane, calling it a "disastrous mistake".
The military claimed air defences were fired in error during an alert which was imposed after Iranian missile struck US targets in Iraq.
Iran denied for days after Wednesday's crash that it brought down the plane, although a top Revolutionary Guards commander said on Saturday he had told authorities about the unintentional missile strike the day it happened.
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Speaking from Tehran, Al Jazeera Dorsa Jabbari said, "There is a lot of anger. Iranians are demanding justice and accountability. Many people including families of the victims are in shock. They do not understand why their government would have lied to them for this long."
"Vigils that were held near Amir Kabir University quickly turned into anti-government protests with people calling for the IRGC to leave the country," she said.
Iran's leadership last faced mass protests in November following the rise in petrol prices.
Foreign governments have condemned Iran's action, with Ukraine demanding compensation and a US official calling the downing "reckless".
Britain said Tehran's admission was an important first step and urged a de-escalation in tensions.
Supreme leader Khamenei, until now silent about the crash, said information should be made public, while top officials and the military issued apologies.
But the state television suggested revealing the truth might be used by the "enemies of Iran", usually a reference to the US and Israel.
The crash heightened international pressure on Iran after months of friction with the US and tit-for-tat attacks.
A US drone strike killed a top Iranian military commander in Iraq on January 3, prompting Tehran to fire at US targets on Wednesday.
Experts said mounting international scrutiny would have made it all but impossible to hide signs of a missile strike in any investigation.
They said Iran may have felt a U-turn was better than battling rising criticism abroad and growing grief and anger at home, as many victims were Iranians with dual nationality.
A rare apology
Iran's Revolutionary Guards, in a rare step, apologised to the nation and accepted full responsibility for the plane crash.
Senior Guards commander Amirali Hajizadeh said he had informed Iran's authorities on Wednesday about the unintentional strike, a comment that raised questions about why officials had publicly denied it for so long.
Speaking on state television, he said he wished "I could die" when he heard the news about the incident.
Ukraine demanded an official apology and compensation. Canadian Prime Minister Justin Trudeau called for "a complete and thorough investigation" with Iran's full cooperation.
Iranian Foreign Minister Mohammad Javad Zarif wrote on Twitter that "human error at time of crisis caused by US adventurism led to disaster", citing an initial armed forces investigation into the crash.
A military statement said the plane flew close to a sensitive Revolutionary Guards site at a time of high alert.
But Ukraine said the plane was in a normal flight corridor. Iran's Civil Aviation Organisation also said the airliner had not veered off its normal course.
Ukraine International Airlines said Iran should have closed the airport, adding that it received no indication it faced a threat and was cleared for take-off.
An expert weighs in
Analysts said a probe would almost certainly have revealed signs on the smashed fuselage of a missile strike.
"There's nothing you can do to cover it up or hide it," said Anthony Brickhouse, an air safety expert at Embry-Riddle Aeronautical University and former US National Transportation Safety Board investigator. "Evidence is evidence."
The disaster echoed a 1988 incident, when a US warship shot down an Iranian airliner, killing 290 people. While Washington claimed it was an accident, Tehran said it was intentional.